1. Field of the Invention
The present invention relates to electron beam welding practice and, more particularly, to a device for detecting a seam between abutting workpieces by a scanning electron beam.
The invention can be most advantageously used in control systems for automatically directing a welding electron beam at the seam formed between workpieces to being joined.
2. Description of the Prior Art
Owing to its technological possibilities electron beam welding is finding ever increasing application for joining high-melting and chemically resistant metals, as well as in manufacturing articles from conventional materials such as steels, aluminium alloys, and copper-based alloys.
A quality joint weld can be obtained provided that, apart from meeting requrements of current parameters of the scanning electron beam, the electron beam is properly positioned relative to the seam. It is quite evident, at the sametime, that because of various irregularities in seams to produce a high-quality joint weld is impossible without accurately positioning and moving the electron beam along the seam. Special features of electron beam welding at high temperatures and speeds of treating articles, which make visual control over the welding process extremely inconvenient, on the one hand, and ever growing application of the electron beam welding on the other, account for the ever increasing importance of systems of automatically directing the electron beam at the seam.
Precision of automatically directing the welding electron beam at the seam being formed depends to a great extent on the degree of accuracy of means used for automatically detecting and tracking the seam. Prior art seam detecting devices can be divided into four main groups: mechanical, electromechanical, photoelectrical, and electronic. The mechanical seam detecting devices, though simple in construction, feature a low degree of accuracy, which is responsible for their limited application.
Higher degree of accuracy and efficiency are displayed by the electromechanical seam detecting devices. For instance, a system for automatically directing the welding electron beam at the seam formed between the articles being welded, developed by the company "Sciaky" (France), employs a mechanical tracer for tracking the seam formed between the articles being joined, which tracer being mounted on the electron gun so that the tracer pin is located ahead of the electron beam, and an information electric signal generating unit for producing a signal indicative of variations in the workpiece surface. This information electric signal considerably improves the degree of accuracy of directing the welding electron beam at the seam and widens functional possibilities of the systems in which it is employed, for instance, by employing electronic computers. However, combination of mechanical and electrical (and even electronic) devices is associated with considerable difficulties. In addition, such a combination inevitably causes loss of information and time both in the mechanical units and interaction between mechanical devices and electrical circuits.
Application for the above purpose of the photoelectric seam detecting means encounters the same difficulties as in the case of electromechanical means. The necessity of maintaining the required transparency of lenses during the welding process is a further disadvantage of the photoelectric systems. This disadvantage is that metal vapours produced by the electron beam impingement upon the workpiece surface settle on the lenses, thereby obscuring the image of the article being welded.
All the above systems for automatically directing the welding electron beam at the seam require various additional operations before welding proper such as, for example, treatment of edges, arrangement of reflecting line along the seam, etc.
More promising at present are systems of automatically directing the welding electron beam at the seam, employing electronic means for tracking the seam. Such apparatus have a number of advantages over the above-described systems. Thus, the information on position of the welding electron beam relative to the seam between the workpieces being joined is obtained with the aid of the electrons reflected from the surface of the workpiece as a result of the electron beam impingement upon this surface, which electrons are conventionally termed as secondary electrons. The intensity of the secondary electrons flow is known to depend on the electron beam current and the surface variations of a workpiece being welded. These surface variations are detected by scanning the workpiece surface with the aid of a scanning electron beam. When the electron beam crosses the seam, the intensity of the secondary electrons flow varies in response to which an electric pulse is formed which is termed as an information signal indicative of variations in the workpiece surface. The scanning electron beam is generated either by a special tracking electron beam gun or by a welding electron beam gun, in which case the welding electron beam gun is alternately switched from welding to scanning mode of operation.
The U.S. Pat. No. 3,426,174 describes a device for tracking a seam between workpieces being joined using a scanning electron beam. The device comprises a means for generating a scanning beam, connected to a power supply source, and an electron back scatter detector disposed in a shield and positioned during welding between the scanning electron beam generating means and the workpiece to be welded. The back scatter detector has an aperture intended for beam passage therethrough, and a tap. Connected to the tap is a resistor across which there is formed an information signal, which resistor functions as a generator of information signals indicative of surface variations, and in particular of the presence of the seam in the scanning electron beam impingement zone. These information signals are used for directing the electron welding beam at the seam between abutting workpieces being joined. The detector is shielded to prevent its being influenced by stray electrons as, for example, backscattered from an adjacent welding beam.
A display error of the above device depends on the error in forming the signal indicative of variation in workpiece surface conditions, as well as on the other errors, for instance, such as instrumental errors of the electron beam deflection system, etc. The degree of accuracy of the device is also affected by electromagnetic fields produced as a result of interaction of the welding electron beam with the metal vapours, and by a time constant of the signal generator constructed as a resistor. The shape of the signal indicative of surface variations is not protected from influence of the scanning electron beam current pulsations. Though the shield accommodating the detector protects to a great extent the device operation from the enfluence of electromagnetic fields, complete elemination of the electromagnetic field influence can be achieved provided the shield entirely envelops the detector, in which case such a shield would be in contact with the workpiece being welded. This, of course, would considerably limit the technological possibilities of the device as a whole.